Urethane acrylates based on caprolactone-modified resins are known from U.S. Pat. No. 4,188,472, for example. In DE 2939584 (=U.S. Pat. No. 4,188,472), 2-hydroxyethyl acrylate is reacted ring-openingly with epsilon-caprolactone in the presence of various catalysts based on titanium or tin or on organic acids (sulfuric acid, p-toluenesulfonic acid), and the resulting product is subsequently reacted with diisocyanates to form the urethane.
U.S. Pat. No. 7,022,778 discloses reacting 2-hydroxyethyl acrylate with epsilon-caprolactone in the presence of zinc octoate. The product of the reaction is used as a reactive diluent in two-component polyurethane coating materials.
A disadvantage of these reaction regimes is that the catalysts disclosed result in nonuniform distribution of the addition product.
The organotin catalysts used result in a particular molecular weight distribution on reaction of hydroxyalkyl acrylates with caprolactone. In the course of this reaction, unreacted hydroxyalkyl acrylate is left, and reaction products of hydroxyalkyl acrylates with up to 10 caprolactone units are formed. However, since products with 3 or more caprolactone units show a tendency toward crystallization, their concentration ought not to be too high. Furthermore, the reaction products of the unreacted hydroxyalkyl acrylates with diisocyanates give rise to urethane acrylates of low flexibility.
In order to conform to this requirement, the viscosity of the first stage, i.e., in the reaction of hydroxyethyl acrylate with 2 mol of caprolactone, ought to be situated within a viscosity range of 60-90 mPas (measured using an Epprecht cone/plate viscometer (Cone B) at 23° C.
US 2011/039971 A1 discloses reacting a commercially available addition product of epsilon-caprolactone with 2-hydroxyethyl acrylate with further components onto trifunctional polyisocyanates to form the urethane.
Catalysts referred to for the formation of the urethane include, in general form, dibutyltin laurate, bismuth carboxylate, or zirconium chelates. The matter of which catalyst is explicitly used in the examples remains open.
U.S. Pat. No. 6,534,128 discloses reacting a commercially available addition product of epsilon-caprolactone and 2-hydroxyethyl acrylate with further components with diisocyanates to form the urethane.
Catalysts referred to for the formation of the urethane include, in general form, dibutyltin dilaurate, other organotin compounds, and organobismuth and organozirconium compounds. The examples use exclusively dibutyltin dilaurate as catalyst.
A disadvantage of these reaction regimes is that the commercially available product still contains traces of the catalyst from the preparation of the addition product, and then further catalyst is added to the reaction for the formation of the urethane as well.